01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - In a large bowl, beat the softened butter and powdered sugar together until light and fluffy. Add the egg yolk and vanilla extract; mix until fully incorporated.
03 - Sift in the all-purpose flour and salt. Mix until a soft, cohesive dough forms.
04 - Roll tablespoon-sized portions of dough into 1-inch balls, place on prepared baking sheets about 2 inches apart, and gently flatten each with your palm or the back of a glass.
05 - Bake for 10 to 12 minutes, just until the edges begin to turn golden. Allow cookies to cool completely on a wire rack.
06 - In a food processor, pulse pistachios and granulated sugar until finely ground. Add melted white chocolate, butter, and 2 tablespoons heavy cream; process until smooth, adding an extra tablespoon cream if needed for spreadable consistency.
07 - Spread a generous dollop of pistachio cream onto the flat side of half the cookies. Top with the remaining cookies, pressing gently to create sandwich-style cookies.
08 - If desired, roll the edges of the filled cookies in chopped pistachios for a refined finish.
09 - Store cookies in an airtight container at room temperature for up to 3 days.